1. Field of the Invention
The present invention relates to a magnetic recording medium and a method of fabricating the same and, more particularly, to a magnetic recording medium formed by a method of heating a substrate when forming a magnetic film thereon. The present invention also relates to a high-quality magnetic disk capable of high-density recording, and a magnetic disk unit for using the magnetic disk.
2. Description of the Related Art
Intensive efforts have advanced in recent years in the field of magnetic disk units to increase recording density for disks of various diameters. Important factors in the increase of recording density are the improvement of coercive force, reduction of noise attributable to the recording medium, and reduction of flying height of the magnetic head relative to the medium. There is a correlation between coercive force and the temperature at which the substrate is heated during the formation of the magnetic recording medium. Particularly, when the substrate is heated at a high temperature, the coercive force is high.
However, when the substrate is heated at a high temperature, it is subject to warping, which makes it difficult to achieve a high coercive force for the resulting recording medium. In fact, if the substrate warps greatly, undulations are superposed on the output waveform during playback, or, in extreme circumstances, the head may crash against the substrate as a result of the warp. Consequently, it is difficult to reduce the flying height.
Additionally, it is important that the magnetic disk have a uniform in-plane characteristic for high-density magnetic recording.
When fabricating a magnetic disk, a substrate is heated in a vacuum and thin films, including a magnetic film and a protective film, are formed on the substrate by a sputtering process (most often used to produce a rigid, or hard disk) or an evaporation process (most often used for creating a floppy disk). The properties of the magnetic thin film are greatly dependent on the temperature of the substrate during the film forming process. Thus, to form a thin film having a uniform quality, it is desirable to heat the substrate so that the distribution of temperature on the substrate surface is uniform. If the substrate is heated irregularly at a high rate, the substrate may warp or crack, potentially causing the substrate to fall off its support in the vacuum vessel; at least, the substrate will be defective.
Japanese Patent Laid-Open Nos. 2-43360; 2-179879; 3-6367; and 5-144557 disclose methods of regulating substrate temperature distribution. Laid-Open Nos. '360 and '367 use a sectional heater, while Laid-Open Nos. '879 and '557 disclose a heating plate between a heater and the substrate to heat the substrate uniformly. Japanese Patent Laid-Open No. 5-33128 discloses a moving heater for heating a glass substrate uniformly.
Methods using a fixed heater have suffered from ineffective heat distribution, which worsens as the interval between the substrate and the heater increases. Consequently, film properties, such as coercivity, vary over the surface of the substrate. Moreover, if the heat distribution on the substrate varies greatly, which is more likely if a high rate of heating is employed, the substrate warps. Further, if the substrate is composed of a material having a low heat conductivity, such as glass, the substrate may crack if the range of heat distribution is large.
Another disadvantage of the method disclosed in JP '360 is that a sectional heater requires a plurality of external controllers and a complicated heating process. JP '128 suffers from the further disadvantage that a heater-moving mechanism is required which has a large heating chamber and a complicated construction.
Currently, substrates heated by conventional methods during the formation of magnetic disks result in magnetic disks having a coercive force in the range of about 1500 to about 1600 Oe. Such magnetic disks are unsuitable for the high-density recording to be achieved with future technology.